A comprehensive background discussion of mechanical ventilation can be found in “Principles and Practice of Mechanical Ventilation” (1994) Edited by Martin J Tobin, published by McGraw-Hill Inc., ISBN 0-07-064943-7.
The use of nasal Continuous Positive Airway Pressure (CPAP) to treat Obstructive Sleep Apnea (OSA) was invented by Colin Sullivan, see U.S. Pat. No. 4,944,310. Generally, the treatment involves providing a supply of air or breathable gas from a blower to a patient via an air delivery conduit and a patient interface. While treatment is effective, some patients find it uncomfortable. Improving patient comfort and compliance is a continuing challenge.
One way to improve comfort is to provide a more comfortable patient interface. In this regard, the ResMed MIRAGE™ masks have provided significant improvement in comfort. See U.S. Pat. Nos. 6,112,746; 6,357,441; 6,581,602 and 6,634,358. A more recent development is the ResMed MIRAGE™ ACTIVA™ mask series. See International Patent Application WO 2001/97893.
In the early days of nasal CPAP systems for treating OSA, patients were first titrated in a clinical study to determine an optimal treatment pressure. Titration involves a patient sleeping overnight in a clinic and being tested with a mask and CPAP device. The treatment pressure provided by the CPAP device is adjusted until apneas are eliminated. The treatment pressure is usually in the range of 4-20 cmH20. A device would be set to that pressure and given to the patient to take home. A subsequent development was the automatically adjusting device that a patient could take home. The automatically adjusting device will raise and/or lower the treatment pressure based on indications of obstructive sleep apnea, such as snoring. Such devices are sometime generically referred to as Automatic Positive Airway Pressure (APAP) devices. See U.S. Pat. Nos. 5,245,995; 6,398,739; and 6,635,021.
Another type of nasal CPAP device provides a first pressure during inhalation (sometimes termed an IPAP) and a second, lower pressure during exhalation (sometimes termed and EPAP). Examples of these include the ResMed VPAP™ series, and the Respironics BiPAP series. Bilevel CPAP devices may be prescribed for patients who do not comply with single pressure CPAP devices. Some patients perceive that the lower pressure during exhalation is more comfortable, at least while they are awake. A difficulty with these devices is deciding how to set the IPAP and EPAP pressures. If the EPAP is too low it may be insufficient to prevent obstructions, hence some clinicians may set the EPAP pressure to the pressure titrated during the sleep study. In those patients, the IPAP pressure, and thus the average pressure will be higher that that required to eliminate apneas.
Another form of automatically adjusting CPAP device is the ResMed AUTOSET™ SPIRIT™ device. In this device, the CPAP pressure is automatically increased or decreased in accordance with indications of flow limitation, such as flow flattening, snore, apnea and hypopnea. See U.S. Pat. Nos. 5,704,345; 6,029,665; 6,138,675; and 6,363,933. An advantage of an automatically adjusting system is that over time the treatment pressure required may vary for a particular patient and a correctly functioning automatic system can obviate the need the patient to return for a subsequent sleep study. These patents also describe a method and apparatus for distinguishing between so-called “central” and obstructive apneas. The contents of all of the aforesaid patents are incorporated by cross-reference.
Another device for treating certain types of Sleep Disordered Breathing such as Cheyne-Stokes Respiration (CSR) is the ResMed AutoCS™ device. Among other things, this device provides a supply of air or breathable gas with a smooth comfortable waveform, sophisticated tracking of the patient's respiratory phase, and servo-control of patient ventilation. See U.S. Pat. Nos. 6,484,719; 6,532,957; and 6,575,163 (the “AutoVPAP” patents). See also U.S. Pat. No. 6,532,959. The contents of these patents are all incorporated by cross-reference.
Some OSA patients find treatment with the above devices uncomfortable and they become non-compliant with the therapy. Other patients such as cardiovascular patients with Congestive Heart Failure, patients with REM Hypoventilation, and patients with Respiratory Insufficiency could also benefit from a more comfortable and/or effective form of therapy.